Aerocaribe has discontinued flights between Belize and Cancun. This is typical for the off-season, and flights probably will be restored after Thanksgiving (but who knows for sure?) I don't recall the exact prices, but it's fairly expensive, something like US$130 one-way. If all else fails, you can fly or take the new ferry to Corozal Town and hop over the border to Chetumal and catch a flight there. Or a bus, which in Mexico is more comfortable than the plane.